Talking about lack of confidence as “syndrome” puts the focus (and potentially, blame) on yourself. But of course, there are both sides. A lot of what holds you back when taking on new things isn’t just you — it’s the culture and team you’re in. It’s very easy to have a few people whose subtle cues (rolling eyes, “oh, you don’t know that?”, pontificating, begrudging time to answer questions, etc.) deeply affect how others on the team think and feel. Junior or less “alpha” engineers quiet down, don’t ask questions, are unhappy, and the whole team is less effective.
On the bright side, this is fixable! In small startup engineering teams, one thing I’ve found that helps is actively mentoring people to mentor. Often, directly talking about mentoring with the more talented, senior people on the team (who sometimes are the ones with these habits) really improves culture. It’s just a matter of helping them to understand the effects of their actions, and that since they are senior, it’s their job to spend half an hour a day mentoring, and it’s their job to make others feel appreciated and grow in abilities.
A good motto is, assume everyone starts out with 100% intelligence and 0% expertise. If you’re great at what you do, use some of your own intelligence and expertise to help others increase that second number.